This application is based on Japanese Patent Applications No. 2001-215821 filed on Jul. 16, 2001 and No. 2001-232531 filed on Jul. 31, 2001 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a controller for an internal combustion engine and, more particularly, to a controller for an internal combustion engine, for controlling an air-fuel ratio of a mixture supplied to an internal combustion engine.
2. Related Art
Conventionally, an air-fuel ratio control of making the air-fuel ratio of a mixture supplied to an internal combustion engine (hereinbelow, also simply called an engine) coincide with a predetermined value is known. The air-fuel ratio control is effective to decrease a fuel consumption of an internal combustion engine and reduce emission of the internal combustion engine.
For example, a lean combustion control of setting the air-fuel ratio of an engine to a value on the lean side with respect to a stoichiometric air-fuel ratio is known. Particularly, a control of setting the air-fuel ratio to a value close to the lean limit of the engine is called a lean limit control. Under the lean limit control, the engine is operated at the leanest air-fuel ratio at which fluctuations in the revolution speed of the engine can be maintained at a predetermined value or less. Concretely, fluctuations in revolution is detected and a fuel injection amount is corrected by being decreased so as not to exceed a target revolution fluctuation. The target revolution fluctuation is set to a value at which drivability does not deteriorate. As a result, the air-fuel ratio is controlled to the leanest side within the range in which the fluctuation of revolution does not exceed the target value, that is, to a value close to the lean limit. In the lean limit control, however, hunting may occur in fluctuations of revolution in relation to the fuel injection amount. The hunting deteriorates drivability. Such a problem occurs in a control of correcting the fuel injection amount in accordance with engine speed.
On the other hand, the lean combustion is also executed at the time of a cold start of the engine. However, at the time of start, to start the engine reliably and stabilize the revolution of the engine early, special consideration is necessary. For example, at the time of a cold start, generally, the fuel injection amount is increased. However, when the lean combustion is performed, torque becomes insufficient. Consequently, the fuel injection amount is corrected to be decreased on the basis of a deviation between a reference engine speed and an actual engine speed in consideration of insufficient torque due to the lean combustion. In the control based on the deviation, however, drivability may deteriorate and, moreover, the problem of hunting also arises.
Further, there is a case that the air-fuel ratio of the engine is controlled on the basis of a signal from a sensor. For example, an O2 sensor for outputting a signal which is inverted at a predetermined air-fuel ratio in accordance with oxygen concentration or an A/F sensor for outputting a linear signal indicative of an air-fuel ratio in accordance with oxygen concentration is used. In such a control, control accuracy of the air-fuel ratio depends on the accuracy of a sensor signal. Consequently, when a sensor temporarily or continuously outputs an inaccurate signal having an error, the air-fuel ratio of the engine cannot be controlled properly.
An object of the invention is to provide a controller for an internal combustion engine, capable of suppressing deterioration in drivability.
Another object of the invention is to provide a controller for an internal combustion engine, for controlling the air-fuel ratio to the lean side while suppressing deterioration in drivability.
Further another object of the invention is to provide a controller for an internal combustion engine, for controlling the air-fuel ratio to the lean side at the time of a cold start while suppressing deterioration in drivability at the time of a cold start of an engine.
Further another object of the invention is to provide a controller for an internal combustion engine, capable of preventing an air-fuel ratio from reaching an abnormal value.
Further another object of the invention is to provide a controller for an internal combustion engine, capable of controlling an engine while preventing an excessive error of an air-fuel ratio in the case where a sensor for detecting the air-fuel ratio becomes abnormal.
According to a first feature of the invention, a correction value which suppresses engine speed fluctuations is selected and is used together with a lean correction value to correct the fuel injection amount.
The correction value which is set on the basis of an engine speed fluctuation amount is set to, for example, a value making combustion more stable as the engine speed fluctuation amount increases. As the engine speed fluctuation amount decreases, the fuel injection amount is decreased to set the combustion air-fuel ratio to the lean side. Final correcting means compares a correction value based on the engine speed fluctuation amount with a final correction value of last time and selects one of them as the correction amount which suppresses the engine speed fluctuation. As a result, a control of suppressing deterioration in drivability at the time of performing a lean control can be carried out.
A lean correction value may be set on the basis of a reference engine speed which is set as a reference value of the engine speed and an engine speed detected by engine speed detecting means. In such a manner, the lean correction value can be set in consideration of a torque margin required to set the air-fuel ratio to the lean side.
The engine speed fluctuation detecting means may obtain an angular velocity in each of a plurality of cylinders and detect fluctuations on the basis of variations of the angular velocity.
According to a second feature of the invention, a correction value for correcting a predetermined parameter is set on the basis of an engine speed fluctuation. Either the correction value or a final correction value of last time, which suppresses the engine speed fluctuations is selected and used for correcting the parameter of the internal combustion engine control.
With the configuration, a control can be executed while suppressing deterioration in drivability.
For example, in the case of executing an ignition timing retarding control for a catalyst early warm-up at the time of a cold start of an internal combustion engine, the invention can be applied to correction of an ignition timing.
The invention may be also applied to correction of a timing of opening an intake valve and/or an exhaust valve of a variable valve mechanism.
In the case where the target air-fuel ratio of lean combustion is set to a predetermined A/F value as shown by the solid line of FIG. 15, when the combustion air-fuel ratio becomes on the lean side, the engine speed fluctuation increases. In contrast, when the ignition timing becomes on the advance side as shown by the broken line and the alternate long and short dash line in the diagram, the air-fuel ratio at which the engine speed fluctuation increases shifts to the lean side with respect to the solid line. Consequently, when the retarded ignition timing for a catalyst early warm-up is set to a normal ignition timing on completion of the catalyst early warm-up or the like, since combustion performed at the ignition timing controlled on the advance side is more stable than combustion performed at the ignition timing controlled on the retard side, the engine speed fluctuation decreases.
Usually, in the case of controlling the target air-fuel ratio of lean combustion to a predetermined A/F value, if there is no means for detecting the air-fuel ratio such as an air-fuel ratio sensor at the time of a cold start, the fuel injection correction amount is computed on the basis of the engine speed fluctuation amount. However, when the engine speed fluctuation decreases due to completion of the catalyst early warm-up or the like as described above, the correction amount which is set on the basis of the engine speed fluctuation amount is set to the correction amount used for combustion at a lean air-fuel ratio. There is consequently possibility that combustion is carried out at a leaner air-fuel ratio.
According to a third feature of the invention, a basic fuel injection amount is corrected on the basis of an engine speed fluctuation amount by roughness correcting means, and the correction amount in the roughness correcting means is corrected on the basis of the value of a parameter exerting an influence on the engine speed fluctuation amount.
Thus, the air-fuel ratio is prevented from becoming leaner during a lean control.
As the parameter exerting an influence on the engine speed fluctuation amount, at least one of an ignition timing for performing a catalyst early warn-up in a cold start operation of the internal combustion engine, revolution speed of the internal combustion engine, and an intake air amount supplied to the internal combustion engine can be used.
According to a fourth feature of the invention, a guard value for a feedback correction factor is set on the basis of fluctuations in revolution speed of the internal combustion engine. In this specification a guard value is an upper limit or a lower limit, and a value is regulated by the guard value.
Thus, the actual air-fuel ratio can be suppressed from being deviated from a target air-fuel ratio.